Directional microphones are well known for use in speech systems to minimise the effects of ambient noise and reverberation. It is also known to use multiple microphones when there is more than one talker, where the microphones are either placed near to the source or more centrally as an array. Moreover, systems are also known for determining which microphone or combination to use (i.e. higher noise and reverberation requires that an increased number of directional microphones be used). In teleconferencing situations, it is known to use arrays of directional microphones associated with an automatic mixer. The limitation of these systems is that they are either characterised by a fairly modest directionality or they are of costly construction.
Microphone arrays have been proposed to solve the foregoing problems. They are generally designed as free-field devices and in some instances are embedded within a structure. The limitation of prior art microphone arrays is that the inter-microphone spacing is restricted to half of the shortest wavelength (highest frequency) of interest. This means that for an increase in frequency range, the array must be made smaller (thereby losing low frequency directivity) or microphones must be added (thereby increasing cost). The other problem with this approach is that the beamwidth decreases with increasing frequency and side lobes become more problematic. This results in significant off axis “coloration” of the signals. As it is impossible to predict when a talker will speak, there is necessarily a time during which the talker will be off axis and this “coloration” will degrade the signal.
It is an object of this invention to provide a microphone array having a reasonably constant beampattern over a frequency range that extends beyond the traditional limitation of inter-sensor spacing to half a wavelength.
The following references illustrate the known state of the art:    [1] Michael Brandstein, Darren. Ward, “Microphone arrays”, Springer, 2001.    [2] Gary Elko, “A steerable and variable first-order differential microphone array”, U.S. Pat. No. 6,041,127, Mar. 21, 2000.    [3] Michael Stinson, James Ryan, “Microphone array diffracting structure”, Canadian Patent Application 2,292,357.    [4] Jens Meyer, “Beamforming for a circular microphone array mounted on spherically shaped objects”, Journal of the Acoustical Society of America 109 (1), January 2001, pp. 185-193.    [5] Marc Anciant, “Modélisation du champ acoustique incident au décollage de la fusée Ariane”, July 1996, Ph.D. Thesis, Université de Technologie de Compiègne, France.    [6] A. C. C. Warnock & W. T. Chu, “Voice and Background noise levels measured in open offices”, IRC Internal Report IR-837, January 2002.    [7] S. Dedieu, P. Moquin, “Method for Broadband Constant Directivity Beamforming for Non Linear and Non Axi- Symmetric Arrays Embedded in an Obstacle”, U.S. Patent Application Publication No. 2004/0120532.    [8] Morse and Ingard, “Theoretical Acoustics”, Princeton University Press, 1968.
Brandstein and Ward [1] provide a good overview of the state of the art in free-field arrays. Most of the work in arrays has been done in free field, where the size of the array is necessarily governed by the frequency span of interest.
The use of an obstacle in a microphone array is discussed in Elko [2]. Specifically, Elko uses a small sphere with microphone dipoles in order to increase wave-travelling time from one microphone to another and thus achieve better performance in terms of directivity. A sphere is used since it permits analytical expressions of the pressure field generated by the source and diffracted by the obstacle. The computation of the pressure at various points on the sphere allows the computation of each of the microphone signal weights. The spacing limit is given as 2λ/π (approx. 0.64λ) where λ is the shortest wavelength of interest.
M. Stinson and J. Ryan [3] extend the principle of microphone arrays embedded in obstacles to more complex shapes using a super-directive approach and a Boundary Element method to compute the pressure field diffracted by the obstacle. Stinson and Ryan emphasise low frequency, trying to achieve strong directivity with a small obstacle and a specific treatment using cells (i.e. reactive impedance) thereby inducing air-coupled surface waves. This results in an increase in the wave travel time from one microphone to another and increases the “apparent” size of the obstacle for better directivity at low frequencies. Stinson and Ryan have proven that using an obstacle provides correct directivity in the low frequency domain, when generally other authors use microphone arrays of large size. Additionally Stinson and Ryan invoke the use of acoustic absorbent materials to provide impedance treatment. However, the application is designed for narrow band telephony.
The benefit of an obstacle for a microphone array in terms of directivity and localisation of the source or multiple sources is also described in the literature by Jens Meyer [4] and by Marc Anciant [5]. Jens Meyer demonstrates the benefit of adding a sphere on a microphone array compared to a free-field array in terms of broadband performance and noise rejection. Anciant describes the “shadow” area for a 3D-microphone array around a mock-up of the Ariane IV rocket in detecting and characterising the engine noise sources at take-off.
With the exception of Elko [2] (who sets the spacing limit at 2λ/π), the prior art explicitly or implicitly concedes the requirement for a high frequency performance limit defined by an inter-element spacing of λ/2 to avoid grating lobes in free-field.
The superdirective beamformers that are commonly used for microphones are discussed in chapter 2 of Brandstein [1] and the essential elements are noted below, to better understand the background of the present invention.
Beamforming may be used to discriminate a source position in a “noisy” environment at a frequency ω in a band [ω0, ωn]. Let d(ω) be the signal vector containing the signal di(ω) of each microphone of the array when the source is active. Let n(ω) be the vector of noise signal at each microphone and Rnn(ω) the noise correlation matrix. Depending on the environment, this matrix can be defined in different ways, such as for diffuse spherical or cylindrical isotropic noise or more simply for white noise. Reference [5] provides a detailed discussion of how the noise correlation matrix may be defined.
Beamforming consists of finding a vector wopt(ω) of coefficients wi(ω) such that weighting the signal di(ω) at each microphone with each wi(ω) creates a beam towards the source. For a super directive approach, the problem can be written in the following way:
                                                                        Min                w                            ⁢                              1                2                            ⁢                              w                H                            ⁢                                                          ⁢                              R                nn                            ⁢              w                                                                                                                    subject              ⁢                                                          ⁢              to                                                                                                                                                      w                  H                                ⁢                                                                  ⁢                d                            =              1                                                          (        1        )            where the dependency in ω has been omitted for clarity purposes.
The optimal weight vector is:
                              w          opt                =                                            R              nn                              -                1                                      ⁢            d                                              d              H                        ⁢                          R              nn                              -                1                                      ⁢            d                                              (        2        )            
As described in U.S. Patent Application Publication No. 2004/0120532, linear or quadratic constraints can be added to impose a specific pattern to a beam, to reduce the coupling between the microphone beam and a loudspeaker or to keep the beam constant vs. frequency or vs. angle when the obstacle is not axi-symmetric.